CA1042067A - Flat alkaline cell construction - Google Patents

Abstract

FLAT ALKALINE CELL CONSTRUCTION

Abstract of the Disclosure A flat alkaline cell is described which comprises an electrode assembly including at least a pair of flat electrode elements of opposite polarity, a porous separator containing the alkaline electrolyte interposed between and in contact with the pair of electrode elements and a current collector disposed adjacent to and in electrical connection with one of the pair of electrode elements at one end of the electrode assembly, wherein the electrode assembly is enclosed within a sealed, liquid impervious plastic film envelope having an opening in one wall thereof which exposes the current collector for making external electrical connection, and wherein a layer of an adhesive sealant which is non-wettable by the alkaline electrolyte, tightly adheres and seals together the current collector and the wall of the envelope at least around the periphery of the opening.

Description

10420~ 8894 Back~round of the Invention This invention relates to flat, electric current-producing cells using an alkaline electrolyte, and more particularly to improvements in flat alkaline cells of the type wherein the active cell elements are enclosed within a sealed envelope made of a liquid impervious, plastic film.
Alkaline cells of both the primary and secondary type are well knawn. Alkaline cells in general po6sess a number of outstanding advantages over cells using other types of electrolytes such as the more familiar Leclanche dry cell using an ammonium chloride electrolyte. One advantage of alkaline cells is that they exhibit a high current discharge capacity under continuous load for a relatively long period. Moreover, alkaline cells have a substantially more level voltage discharge curve.
Additionally, they have a relatively high ratio of energy ~o cell volume and are not as adversely affected by changes in temperature. Typical examples of primary . .
-; 20 alkaline cells are those utilizing electrode systems of -` zinc/manganese dioxide, zinc/mercuric oxide and zincj ` silver oxide. Secondary or rechargeable alkaline cells may also be constructed utilizing a number of different electrode sy~tems. Of course, the most familiar of these cells i8 the rechargeable nickel-cadmium cell.

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:: . ,, ~; . . -~O~ZOti7 There is an increasing demand today in the electronics industry for various types of alkaline cell batteries. Alkaline cell batteries, for instance, are ideally suited for use as the power source in many portable electronic devices such as electronic calculators, radios, television sets, tape recorders and the like. For most of these applications, flat alkaline cell batteries would be preferred because of their potentially higher capacity per unit volume as compared to conventional round or button cell batterieg. Thi~ is an important consideration since most portable electronic devices today could be made more -~ compact if less space were required to accommodate the batteries~
One flat cell construction that has been used successfully with electrolytes other than alkaline electro-lytes is the type wherein the active cell elements are enclosed within a sealed envelope made of a liquid impervious plastic film. Such a cell envelope may be madej for example, from two plastic films which are joined tightly together around the marginal borders thereof such as by heat sealing. Suitable external connections with the cell electrodes may be made, for example, by an electrical lead extending through the marginal border seal or through the provision of an opening in the plastic film envelope which exposes an electrode or a current collector such as a thin metal . . .

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lO~ZO~7 foil. Numerous flat cell construction~ of this type are known in the art, reference being made for example to Soltis, U.S, Patent No. 2,870,235 and Nagorski, U.S.
Patent No. 2,847,495.
Various attempts have been made to manufacture flat cell constructions of the type described utilizing an alkaline electrolyte. So far, however, these attempts have failed ma~nly due to the difficulty in sealing such - flat cell constructions against leakage of alkaline electrolyte. Generally, cells utilizing an alkaline electrolyte are the most difficult to seal owing to the ability of the electrolyte to readily wet most surfaces, including both plastic and metal surfaces, and actually creep through the interface between a seal and certain parts of the cell.
In Tamminen, U.S. Patent No. 3,708,340, a flat cell construction of the type described utilizing an alkaline electrolyte i8 disclosed. In this flat cell - construction, external connection with one of the electrodes iB attained through the provision of an opening in the plastic film envelope. A seal composed of a sticky and water-repellent material, such as soft microcrystalline wax, polyisobutylene or silicone grease, - surround~ the immediate periphery of the opening at the interface between one of the electrodes and the plastic - film. The problem with this flat cell construction, ' , . :

~0420G7 8894 however, is that the seal, although made of a water-- repellent materisl, is nonetheless penetrated by the alkaline electrolyte and leakage occurs through creepage of the electrolyte past the seal and out through the opening in the plastic film envelope.
It is therefore the principal object of this invention to provide an improved flat cell construction utilizing an alkaline electrolyte.
A more ~pecific object of the invention is to provide a flat alkaline cell construction which can be effectively sealed against leakage of the alkaline electrolyte.
Still another object of this invention is to provide a flat alkalinelcell construction which can be readily assembled and which is economical to manufacture.
Summary of the Invention ; This invention resides in a flat alkaline cell construction comprising an electrode assembly including at least a pair of flat electrode elements of opposite polarity, a porous bibulous separator containing the ; alkaline electrolyte, interposed between and in contact with the pair of electrode elements, and a current -~ collecto~ which may be a flat ~hin metal plate or foil, disposed adjacent to and in electrical connection with at least one of the pair of electrode elements at one end i ~5~

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~0~ ZO ~7 8894 of the electrode assembly. All of the cell elements are enclosed within a sealed envelope made of a liquid impervious plastic film and preferably a plastic film which is electrically non-conductive. The sealed envelope is formed with an opening adjacent to the current collector which exposes the collector for making external electrical connection, for example, between adjacent cells in a stack of flat cells forming a battery. ~n adhesive sealant which is non-wettable by the alkaline electrolyte, surrounds the periphery of the opening at the interface between the current collector and the plastic film. Preferably, the adhesive ~ealant is applied to the entire face of the current collector in order to tightly seal the plastic film to the collector and also to extend substantially the leakage path and thereby increase the resistance to creepage of the alkaline electrolyte from its location inside the cell. The adhesive sealant preferably used in the practice of the invention is a sealant formulated from the general class of compounds known as "fatty polyamides".
The fatty polyamide used in the sealant material preferably should have an amine number of above about 9.
Although the invention is widely applicable to flat alkaline cell constructions in general, it will be more fully described hereinafter with particular reference to a rechargeable nickel-cadmium cell. Such a rechargeable nickel-cadmium cell comprises a positive electrode containing '~' ~ ,,: ' ;

104~067 8894 an electrochemically oxidizable active material such as nickel hydroxide, a negative electrode containing an electrochemically reducible active material such as cadmium oxide or cadmium hydroxide, and a porous bibulous separator containing the alkaline electrolyte, interposed between and in contact with both the positive and the negative electrodes. The positive and negative electrodes will be so balanced electrochemically with the active materials as to prohibit the generation of excessive gas or gases upon overcharge of the cell. In addition, the positive electrode may also incorporate an antipolar mass in order to prevent electrode polarity reversal upon over-discharge. It will be understood, of course, that the flat alkaline cell construction of the invention may utilize other electrode systems such as the zinclmanganese dioxide system as will readily occur to those skilled in the art.
The invention will be hereinfurther described in detsil by reference to the specific embodiments of the flat cell construction illustrated in the accompanying drawings.
Brief Description of the Drawin~s Fig. 1 is a perspective view of one embodiment of the flat alkaline cell construction of the invention;
Fig. lA is a cross-sectional view of the flat cell illustratsd in Fig. 1, taken along the line lA-LA;

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Fig. 2 is a perspective view of another embodiment of the flat alkaline cell construction;
Fig. 2A is a cross-sectional view of the flat cell illustrated in Fig. 2, taken along the line 2A-2A;
Fig. 3 is a perspective view of another embodi-ment of the flat alkaline cell construction;
Fig. 3A is a cross-sectional view of the flat cell illustrated in Fig. 3, taken along the line 3A-3A;
Fig. 4 is a perspective view of still another embodiment of the flat alkaline cell construction;
Fig. 4A is a cross-sectional view of the flat cell illustrated in Fig. 4, taken along the line 4A-4A;
Fig. 5 is a perspective view of a further embodi-ment of the flat alkaline cell construction; and Fig. 5A is a cross-sectional view of the flat cell illustrated in Fig. 5, taken along the line 5A-5A.
- Description of the Preferred Embodiments ; Figs 1 and lA illustrate a rechargeable flat nickel-cadmium cell made in accordance with the invention.
The cell comprises a pO8 itive electrode plate 10, a negative electrode plate 11, and a porous separator 12 containing an alkaline electrolyte sandwiched between and in facial contact with the positive and negative electrode . .
plates 10, 11, forming a conventional electrode stack . assembly. The positive and negative electrode plates 10, 11 and the separator 12 are rectangular in shape and are 1 0 4 Z 0 ~ 7 8~94 of sub~tantially the same size such that the electrode plates 10, 11 and separator 12 are Rtacked congruently.
Both the positlve and negative electrode plates 10, 11 may be sintered type electrode~ fabricAted from a sintered metal plaque which may be made, for example, by sintering : B layer of metal powder, e.g. nickel, onto both sides of an open or porous substrate such as 8 nickel screen, which serves as a mechanlcal support and electrlcal path. The sintered metal plaque 18 impregnated with the electro~
chemically sctive material in accordance with conventional methot~ well known in the art. The ~orous separator 12 containing the alkaline electrolyte may be made from a conventional separatGr material such as a non-woven organic ~ -fiber matte. A preferred type is made from nylon fiber under -the tradename "Pellon", a trademark of Union Carbide CorPora-tion. The alkaline electrolyte used in the cell may be, for example, a 30 percent by weight solution of potassium hydroxide. ;~
At each ent of the electrode stack assembly ~s provided one of a pair of current collector members 13, 14. -The~current collector 13 is poaitioned in contact with the po~itive electrode plate 10 whlle the other current collector 14 i8 po~ltioned in contact with ~he negative ; electrode plate 11. These current collectors 13, 14 are made from an electrlcally conductlve metal, preferably in the form of thin me~sl foll, which i8 lnert to the alkaline electrolyte such a~ nick~l or nickel plated steel.

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~0~06~ 8894 All of the cell elements including the positive and negative electrode plates 10, 11, separator 12 with absorbed electrolyte, and the current collectors 13, 14 are sealed within a liquid-impervious, electrically non-conductive, plastic film envelope 15. The envelope 15 fits tightly around the side walls of the electrode stack assembly and al90 around the opposite ends thereof forming a pair of end walls 16, 17. These end walls 16, 17 do not completely cover the current collectors 13, 14 but rather ~- 10 overlap only the outer margin thereof forming a pair of openings 18, 19 which expose the center of each current collector 13, 14, respectively. As shown in both Figs 1 and lA, a metal terminal lead 20 is secured ~uch as by welding to the exposed center of the current collector 13 and - constitutes the positive terminal for the cell. Similarly, a metal terminal lead 21 is secured to the exposed center of the current collector 14 and constitutes the negative terminal for the cell.
- Substantially the entire outer surface of each current collector 13, 14, except for the exposed center .. . .
thereof, is cdated with a layer of adhesive sealant 22, 23 respectively, in accordance with the invention. The adhesive sealant 22, 23 tightly seals the interface between each of the pair of end walls 16, 17 of plastic film and , the current collectors 13, 14 against leakage of alkaline electrolyte. Suitably, the adhesive sealant 22, 23 should . ~ .

la~z0~7 be an organic resin which will adhesively bond to both the plastic film and metal collectors. Preferably the adhesive sealant employs a fatty polyamide which is chemically resistant to and not readily wet by the alkaline electrolyte.
The adhesive sealant 22, 23 is first applied as a thin layer over the outer surface of each collector 13, 14, prior to assembly.
The envelope 15 is made from a tubular heat -shrinkable plastic film such as a vinyl film. In assembly of the cell, the positive and negative plates 10, 11, ~-separator 12 and current collec~ors 13, 14 are first stacked together in the manner as described above and then inserted inside the heat shrinkable tube with the outer ends of the tube protruding beyond the current - collectors 13, 14. The plastic film tube is then heated and is caused to shrink down tightly around the side walls of the electrode stack and at the same time, the protruding ends of the tube shrink down forming the pair ~ of end walls 16, 17. Additional heat and pressure may be i 20 required to establish the final adhesive bond.
Figs. 2-5 and the corresponding Figs. 2A-5A
inclusive, illustrate a number of other embodiments of the flat cell construction in accordance with the invention.
In all of these embodiments, the cell elements may be basically the same as those just described, that is, the same sintered type positive and negative electrodes may be . ' . .

~0420~7 used together with the same porous, ~ibulous separator containing the alkaline electrolyte, interposed between the electrodes to form the electrode stack assembly. For the sake of convenience, the same reference numerals will denote identical cell elements in the following description.
In the embodiment of the invention illustrated in Figs. 2 and 2A, one of the pair of current collectors 13, 14 used in the previous flat cell is eliminated and replaced with a terminal lead 24, suitably made of a metal foil, which is secured at one end to the negative electrode plate 11, such as by welding. The cell elements are enclosed within a sealed, liquid-impervious, electrically non-conductive, plastic film envelope 25. This envelope 25 is also made from a tubular heat shrinkable plastic film material. In this case, however, the tube is heat shrunk .~
tightly over both ends of the electrode stack assembly.
The open ends of the tube are then heat sealed to one another and also around the protruding terminal lead 24, to form the liquid-tight seams 26, 27.
Similarly, as in the previous flat cell, the envelope 25 has an opening 28 which exposes the center of the current collector 13. A metal terminal lead 29 -is secured such as by welding to the center of the collector 13. Heat shrinking the plastic tube tightly seals the ~ -interface except for the center thereof, between the overlapping portions of the plastic film forming the . -. ~04Z~67 8894 envelope 25 and the current collector 13 against leakage of the alkaline electrolyte. A layer of adhesive sealant 31 is also applied on the surface of the collector and around the foil terminal lead 24 at the point where it protrudes through the liquid-tight seam 27. Preferably, the adhesive sealant used in this embodiment is the same fatty polyamide sealant which is non-wettable by the alkaline electrolyte.
In the embodimRnt of the invention illustrated in Figs. 3 and 3A, the pair of current rollectors 13, 14 are coated on their outer surfaces with a thin layer 32, 33 of a heat sealable organic resin such as polyethylene, except for a small area which is left exposed at the - center of each collector for-making external electrical connection. A thin narrow layer 34, 35 of adhesive sealant, preferably a fatty polyamide sealant, is applied directly over the heat sealable layer 32, 33, around the outer margin of each current collector 13 ? 14. The cell elements are again enclosed within 8 sealed, liquid impervious, electrically non-conductive, plastic film envelope 36.
- This envelope 36 is also made rom a heat shrinkable : plastic film tube but in this case the tube has one closed and one open end. The tube is heat shrunk tightly around the side walls of the electrode stack assembly, except that corresponding with the open end of the tube.
After the tube shrinks down tightly over both ends of the .
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electrode stack forming the end walls 37, 38, the open end of the tube is heat sealed together to form the liquid-tight seam 39. Both end walls 37, 38 are provided with openings 40, 41 through which the center of each current collector 13, 14 is exposed. Terminal leads 42, 43 are secured such as by welding to the center of each collector 13, 14. After the tube is heat shrunk tightly around the electrode stack in the manner as just described, the end walls 37, 38 are sealed directly to the respective 10 heat sealable layers 32, 33 on each of the current collec-tors 13, 14 by application of heat and pressure. The advantage of this flat cell construction is that the attainment of a tight seal at the interface between the plastic film snd the current collectors surrounding the - opening for making external electrical connection is not solely dependent upon the adhesive sealant.
Figures 4 and 4A illustrate another embodiment of the invention wherein the cell elements are sealed within a liquid impervious, electrically non-conductive, 20 plastic film envelope or housing of a somewhat different construction. In this embodiment, the current collectors 13, 14 are similarly coated on their outer surfaces with a thin layer of the adhesive ~ealant 44, 45~ preferably a fatty polyamide sealant, except for the small area left - exposed at the center of each collector. The electrode stack asse~bly is placed inside a rectangular, open ended . ! ~ ............. ; ' . ' ' , : . . . .

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~0~2~7 8894 container 46 which is made of the liquid impervious, electrically non-conductive plastic film. The container 46 includes side walls 47 and a bottom wall 48 and may be made, for example, by vacuum forming a flat sheet of the plastic film material. The electrode stack fits snugly - inside the container 46 whose open end is sealed off ~y a cover 49 also made of the same plastic film material.
The cover 49 has its outer peripheral edges heat sealed to the peripheral edges of the container 46 forming a continuous liquid-tight seam 50. Both the cover 49 and the container bottom wall 48 are provided respectively with openings 51, 52 through which the center of each current collector 13, 14 is exposed. Terminal leads 53, 54 are secured such as by welding to the center of each .- collector. In the final assembly of the cell, both the cover 49 and the bottom wall 48 are heated ln order to assure that the layers of adhesive sealant 44, 45 adhesively bond the film to each of the current collectors 13, 14.
The embodiment of the invention illustrated in Figs. 5 and 5A incorporates still another type of construc-tion for the sealed, liquid impervious, electrically non-conductive cell envelope. As in the previous flat cell illustrated in Figs. 3 and 3A, the current collectors 13, -14 are coated on their outside surfaces with a thin layer 55, 56, respectively, of a heat sealable organic resin, such as polyethylene, except for a small area left exposed . .

` -104Z~67 at the center of each collector. All of the cell elements are enclosed within a sealed composite envelope 57 which in this instance is made from two tubes of a heat shrinkable plastic film material. The first tube is heat shrunk down over the electrode stack assembly in exactly the same manner as described for the cell illus-trated in Figs. 1 and lA forming both side walls 60 and end walls 61, 62. Both end walls 61, 62 overlap only the marginal borders of the pair of current collectors 13, 14 and leave open a substantial portion of each layer 55, 56 of heat sealable resin applied to each current collector 13, 14. After the first tube is heat shrunk around the electrode stack assembly, the end walls 61, 62 are heat i sealed to the layers 55, 56 of heat sealable resin tightly sealing the marginal borders of each current collector 13, 14. The portions of the heat sealable layers 55, 56 which are left open after the first tube is heat shrunk around the electrode stack assembly is then coated with layers 63, 64, respectively, of adhesive sealant, preferably a fatty polyamide, except for a small area coinciding with the center of each current collector 13, 14 which is left ~
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exposed by the layers 55, 56. A second tube of heat shrinkable plastic film is then heat shrunk down over the first tube forming side walls 65, 66 which overlap the end walls 61, 62 formed by the first tube and also overlapping and bonding tightly to the layers 63, 64 of , :. .: : ~ :.:

la4 2~ ~7 8894 adhesive sealant. The overlapping portions of the second plastic tube are formed with openings 67, 68 which coincide with the small area left exposed at the center of each current collector 13, 14. Terminal leads 69, 70 are secured to the exposed center of the collectors 13, 14 such as by welding.
Although not illustrated in the accompanying drawing, a number of flat alkaline cells constructed in accordance with the inventio~ can be readily stacked together, for example, in a columnar form, to constitute a battery of any desired multiple of single cell voltage.
In constructing such a battery, the positive terminal lead from each independent cell may be connected, such as by welding, to the negative terminal lead of an adjacent cell in the stack to form a series-connected battery. Other arrangements are of course possible, such as a parallel or series-paralled connected battery, as will readily occur to those skilled in the art, In flat cells of the invention utilizing the rechargeable nickel cadmium electrode system as in other cell configurations such as cylindrical and rectangular cells, gas generation and the consequent build up of substantial gas pressure inside the cell can occur par-ticularly if the cell is placed on overcharge for long - periods of time. On overcharge, oxygen gas may be liberated at the positive electrode at a faster rate than .-,:
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it can be recombined at the negative electrode leading to a build up of high internal gas pre~sure. In addition, excessive oxygen gas pressures can lead to nearly complete charging of the negative electrode which results in the generation of hydrogen gas from the negative electrode.
The evolution of hydrogen gas further increases the gas - pressure inside the cell since it does not recombine within the cell as does the oxygen.
In flat cells of this invention the plastic film envelope which i8 made, for example, of a vinyl film is flexible and fairly weak. The cell can rupture although without danger, if the internal gas pressure is allowed to build up to any ~ignificant level such as 160 psi~ for example. In order to avoid this problem, it has been found desirable to provide a mechanism for preventing the generation of hydrogen at the negative electrode while at the same time facilitating the recombination of oxygen.
Accordingly, in the preferred embodiment of a rechargeable nickel cadmium flat cell, the positive and negative electrodes are balanced electrochemically with respect to one another such that the capacity of the negative electrode is greater than that of the positive electrode. Preferably the capacity of the negative electrode is at least one and one-half times greater than that of the positive electrode and may be as great as three times the positive capacity.
Commercial plastic films which may be employed in ~ -18-", -1042~D~ 8894 the practice of this invention include those made of the following materials: vinyl polymers and copolymers, polyvinylidene chloride, polyethylene, polypropylene, nylon, polysulfone, polystyrene, and fluorocarbon polymers.
For use with the preferred fatty polyamide adhesive, films made of polyethylene, polypropylene, and vinyl polymers and copolymers are preferred. Regular and shrink-type films are available in these materials. Desired film characteristics include the following: low cost, flexibility, tear and puncture resistance, chemical stability and resistance to alkaline battery electrolyte, hot-formability, low oxygen gas and water vapor transmission rates, and of course strong surface adherence with fatty polyamide or equivalent adhesive. To reduce the gas and water vapor transmission rate of the plastic film,it may be vacuum metallized or otherwise given ~ surface metallic coating on one or both sides providing oif course the film is not made electrically conductive enough to put a parasitic current drain on the cell.
It will be evident from the foregoing that the invention provides a flat alkaline cell construction of the type wherein the active cell elements are enclosed `; within a liquid impervious, plastic film envelope which is ~ effectively sealed against leakage of the alkaline ; electrolyte. One important feature of the invention~ of course, is the use of an adhesive sealant which is chemically ,. -19-.

lQ~Z0~7 resistant to and non-wettable by the alkaline electrolyte, surrounting at least the immediate periphery of the - opening in the plastic film envelope for making external electrical connection. Although there are probably a number of orgflnic compounds which exhibit a non-wetting characteristic when in contact with an alkaline electrolyte, the most preferred edhesive sealant for use in the practice of the invention are the fatty polyamides. Such fatty - polyamide sealants for use in alkaline cells are already dis-closed in U.S.Patent No. 3,922,178, issued to J.Winger, filed on November 25, 1975. As disclosed therein, fatty polyamides are produced by reacting a polybasic acid with a poly-functional amlne. Generally, the fatty polyamides useful in the practice of the invention are those having an amine number of above about 9. The amine number is the number of milligrams of KOH equivalent to one gram of fatty polyamide and is determined by procedures well known in ; the art. The fatty polyamide sealants can be mixed with extender~, modifiers, ant hardeners such as epoxy resins, in order to modify the physical properties of the fatty polyamite. In constructlng flat cells in accord with the -~
lnvention, the fstty polyamide sealant can be appl~ed as a hot-melt or from solutlon in a solvent such as an alcohOl/
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. - . .. - - ' -~04Z067 aromatic hydrocarbon mixture. Among the specific commer-cially available fatty polysmides that sre particularly useful in constructing flat cell~ of the invention ~re those produced under the tradename~ of VERSALON and GENBOND, trademarks of General Mills, Inc. and Swift's Z-610, a trademark of Swift and Company.
It has been found that during the as~embly of flat cells of the invention the preferred adhesive 6ealants, i.e. fAtty polyamide6, do not readily wet some types of plastlc film and accordingly a liquid tight seal between the cupped contsiner snd cover snd the metal collector is somet~me6 dlfflcult to atta~n. This dlfficulty may be overcome and the bond between the plastic film and the collector ~ubstaneially improved if the pl~stic film is f~rst sub~ected to successive heat and corona discharge treatu~nts. The proce8s for treating the plastic film is disclosed and claimed ln our copending Canadian Patent application Ser. No. 221,747, filed on March 10, 1975. In this proces~, the plastic film i6 heated to an elevated tem-; 20 per~ture, about 120C ln the case of a polypropylene film, for a per~od of about one half mlnute, optionally cooling the plastic film to ~mbient temperature ant then sub~ecting Ithe film to a high inten~ity corona discharge in the area of the film where the oetal collector i~ to be adhered.
Another ~port~nt feature of the invention re~ides ~ -~n the provl~ion of a oeal for ~ flat ~lkal~ne dry cell ., D
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: ' 104Z0~7 which is so constructed and arranged as to constitute an extended leakage path to resist creepage of the alkaline electrolyte from its location inside the cell. It will be noted in all of the preferred embodiments of the flat cell described that the adhesive sealant or heat sealable resin is applied as a coating or layer over one entire face of each collector, except for the small area which is left exposed at the center thereof, and tightly bonds the whole interface between the plastic film and the collectors.
Since the alkaline electrolyte can only leak out from the cell by penetrating the adhesive seal over this extended creepage path, there is little likelihood of leakage occuring from inside the cell.
Flat cells of the invention exhibit certain ; other advantages over the prior art and particularly the conventional metal-encased round or cylindrical cells and button cells.
Metal containers and covers are heavy, costly, and wasteful of space. They are not readily adaptable ` 20 to rectangular cells. Such cells withstand high internal pressure, but if they do rupture they can be hazardous.
Multi-cell rectangular batteries composed of an assembly of round unit cells are inherently burdened with unused space.
By contrast the improved plastic film encased flat cells of the invention overcome all of the foregoing .' .

10420~i'7 objections. The plastic film adds little additional bulk and weight. It will withstand reasonable operating pressures and if the film does rupture under pressure no hazard exists. The rectangular embodiments illustrated can be readily fitted as s~ngle cells or multiple cell stacks into rectangular battery compartments or enclosures to provide the full energy density practically attainable in the cell system employed. ~ost of the flat cell embodiments shown can be readily adapted to cell shapes other than square or rectangular, for example, circular, elliptical, hexagonal, triangular as well as a variety of irregular geometric figures. The flat cells may thus be tailored to fit special cavities or holders, thus preserving the high energy density capability of the system.
Primary or secondary alkaline cell systems which are believed to be readily adaptable to the novel fla~
cell constructions herein described include the following:
alkaline-manganese dioxide, silver oxide-zinc, nickel cadmium, nickel zinc, and mercuric oxide-zinc.

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Claims (9)

WHAT IS CLAIMED IS:

1. A flat alkaline cell comprising, in combination, an electrode assembly including at least a pair of flat electrode elements of relatively large surface area and of opposite polarity, a porous separator containing an alkaline electrolyte interposed between and in contact with the pair of electrode elements, a pair of current collectors disposed at opposite ends of said electrode assembly with each abutting a separate one of the pair of electrode elements and having a surface area substantially equal to the surface area of the adjacent electrode, said pair of electrode elements and said current collector being stacked congruently with one another,a sealed, liquid impervious plastic film envelope heat shrunk into engagement around said electrode assembly and said current collectors and having an opening on each opposite end of said electrode assembly which exposes said current collectors for making external electrical connection thereto, and a thin layer of a fatty polyamide organic resin, disposed substantially about the entire surface of each of said current collectors facing said plastic film envelope except for a portion of each current collector which is exposed through said opening, for simultaneously adhesively bonding each of said current collectors to said plastic film envelope in response to the application of heat and for sealing the bonded interface therebetween against leakage from alkaline electrolyte.

2. The flat alkaline cell as defined by claim 1, wherein said sealed envelope is made of a plastic film material selected from the group consisting of polyvinyl chloride, polyvinyl chloride copolymers, polypropylene and polyethylene.

3. The flat alkaline cell as defined by claim 2, wherein said electrode assembly includes at least one flat electrode element containing nickel hydroxide as the positive active material and at least one flat electrode element containing cadmium oxide or cadmium hydroxide as the negative active material.

4. The flat alkaline cell as defined by claim 3, wherein the capacity of said negative electrode element is at least one and one-half times greater than the capacity of said positive electrode element.

5. The flat alkaline cell as defined by claim 1, wherein said plastic film envelope is made from two heat shrinkable plastic film tubes, a first tube which is heat shrunk tightly around the side walls of said electrode assembly with the open ends thereof tightly overlapping the outer margin of each one of said pair of current collectors leaving an open space which exposes the center of said current collectors for making external electrical connection and a second tube which is heat shrunk tightly over said overlapping ends of said first tube and said open space with the open ends of said second tube tightly overlapping the side walls of said first tube, and wherein said second tube has openings in the walls overlapping said open space which expose the center of each one of said pair of current collectors.

6. The flat alkaline cell as defined by claim 5, wherein each one of said pair of current collectors is coated with a layer of a heat sealable organic resin over substantially the entire surface thereof except for the center of each current collector and wherein said layer of adhesive sealant is applied over said coated current collectors within said open space defined by said overlapping ends of said first tube, tightly adhering and sealing together each of said pair of current collectors and the walls of said second tube forming said envelope.

7. The flat alkaline cell as defined by claim 6, wherein a terminal lead is secured in electrical connection with the exposed center of each one of said pair of current collectors.

8. The flat alkaline cell as defined by claim 2, wherein said plastic film envelope comprises a cupped container in which said electrode assembly is placed and a cover sealed to said container around the marginal border thereof and wherein said openings are located within the bottom of said container and within said cover forming the walls of said envelope.

9. The flat alkaline cell as defined by claim 8, wherein a terminal lead is secured in electrical connection with the portion of each of said pair of current collectors which is exposed through said openings.